1. Technical Field
The present invention relates generally to an information recorder, information player, information recording method, information playback method, information recording medium, and a program serving medium, and more particularly to an information recorder, information player, information recording method, information playback method, information recording medium, and a program serving medium, in which a tree-structured hierarchical key distribution method is used to reduce the size of a message, thereby minimizing the load of data distribution when a key such as a master key, medium key or the like has been renewed. More particularly, the present invention relates to an information recorder, information player, information recording method, information playback method, information recording medium, and a program serving medium, in which a key distribution method in which each of a number n of recorder/players is disposed at each of leaves of a tree is used to distribute a necessary key, such as a maser key or medium key, for recording or playback of a content data to or from a recording medium via the recording medium or a communications line and the master key or medium key thus distributed is used by each recorder/player to record or play back the content data.
2. Background Art
With the recent advancement and development of the digital signal processing technology, digital recorders and recording media have been prevailing. With such a digital recorder and recording medium, an image or sound, for example, can be repeatedly recorded and played back without any degradation thereof. Since digital data can be repeatedly copied many times with no degradation of the image and sound qualities, so recording media having digital data illegally recorded therein, if put on the market, will cause the copyrighters of various contents such as music, movie, etc. or legal distributors of the contents to be deprived of profits which would come to the latter if such illegal copying is not possible. To prevent such illegal copying of digital data, various illegal-copy preventing systems have recently been introduced in digital recorders and recording media.
As an example of the above illegal-copy preventing systems, SCMS (Serial Copy Management System) is adopted in the MD (mini disc) drive (MD is a trademark). The SCMS is such that at a data player side, audio data is outputted along with SCMS signal from a digital interface (DIF) while at a data recorder side, recording of the audio data from the data player side is controlled based on the SCMS signal from the data player side, thereby preventing the audio data from being illegally copied.
More particularly, the above SCMS signal indicates that an audio data is a “copy-free” data which is allowed to freely be copied many times, a “copy-once-allowed” data which is allowed to be copied only once or a “copy-prohibited” data which is prohibited from being copied. At the data recorder side, when receiving an audio data from the DIF, SCMS signal transmitted along with the audio data is detected. If the SCMS signal indicates that the audio data is a “copy-free” data, the audio data is recorded along with the SCMS signal to the mini disc. If the SCMS signal indicates that the audio data is a “copy-once-allowed” data, the audio data is converted to a “copy-prohibited” data and the SCMS signal is recorded along with the audio data to the mini disc. Further, if the SCMS signal indicates that the audio data is a copy-prohibited data, the audio data is not recorded to the mini disc. Under a control with the SCMS signal, a copyrighted audio data is prevented from being illegally copied in the mini disc drive unit.
However, the SCMS is valid only when the data recorder itself is constructed to control recording of audio data from the data player side based on the SCMS signal. Therefore, it is difficult for the SCMS to support a mini disc drive not constructed to perform the SCMS control. To apply the SCMS, a DVD player for example adopts a content scrambling system to prevent a copyrighted data from being illegally copied.
The content scrambling system is such that encrypted video data, audio data and the like are recorded in a DVD-ROM (read-only memory) and a decryption key for use to decrypt the encrypted data is granted to each licensed DVD player. The license is granted to a DVD player designed in conformity with a predetermined operation rule against illegal copying etc. Therefore, using the granted decryption key, a licensed DVD player can decrypt encrypted data recorded in a DVD-ROM to thereby play back the video and audio data from the DVD-ROM.
On the other hand, an unlicensed DVD player cannot decrypt encrypted data recorded in a DVD-ROM because it has no decryption key for the encrypted data. In short, the content scrambling system prevents a DVD player not meeting the licensing requirements from playing a DVD-ROM having digital data recorded therein in order to prevent illegal copying.
However, the content scrambling system adopted in the DVD-ROM is directed to a recording medium to which the user cannot write data (will be referred to as “ROM medium” hereunder wherever appropriate), but not to any recording medium to which the user can write data (will be referred to as “RAM medium” hereunder wherever appropriate).
That is to say, copying all encrypted data recorded in a ROM medium as they are to a RAM medium will produce a so-called pirated edition of the data which can be played back by a licensed DVD player.
To solve the above problem, the Applicant of the present invention proposed, as disclosed in the Japanese Published Unexamined Application No. 224461 of 1999 (Japanese Patent Application No. 25310 of 1998), a method in which information to identify each recording medium (will be referred to as “medium ID information” hereunder) is recorded with other data in a recording medium to allow access to the medium ID information in the recording medium only when a player going to play the recording medium has been licensed for the medium ID information.
The above method encrypts data in the recording medium with a private key (master key) acquired through licensing of the medium ID information so that any unlicensed player cannot acquire any meaningful data even if it can read the encrypted data. Note that a player licensed for the medium ID information has the operation thereof restricted against illegal copying.
No unlicensed player can access the medium ID information. The medium ID information is unique to each recording medium. Even if an unlicensed player could copy all encrypted data recorded in such a recording medium to a new recording medium, the data thus recorded in the new recording medium cannot correctly be decrypted by the unlicensed player as well as by a licensed player. Thus, it is substantially possible to prevent data from being illegally copied.
Now it should be reminded that in the above conventional system, a master key stored in a licensed device is generally common to all devices included in the same system. The master key common to a plurality of devices in a system is stored to permit one of the devices to play a recording medium having data recorded therein by any other device in the system (to secure the inter-device operability).
However, if an attacker has succeeded in attacking a device included the in the system and extracted the master key, the encrypted data recorded in the entire system can be decrypted and the entire system will be collapsed. To avoid the above, if it is revealed that an attacking of the device has uncover the master key, the master key has to be renewed to a new one and the new master key has to be granted to all the devices included in the system except for the one having been attacked. This measure can be implemented most simply by giving each of the devices a unique key (device key), encrypting the new master key with each of the device keys to provide a corresponding value, and transmitting the value to each of the devices via a recording medium. However, this will add to the size of to-be-transmitted message proportionally to the number of the destination devices.